1. Field of the Invention
The present invention relate generally to a jet engine testing, and more specifically to failure detection of jet engines on aircraft during all phases of flight.
2. Description of the Prior Art
Many commerical transport aircraft, general aviation aircraft, and military aircraft, are powered by turbojet or turbofan engines. Computers onboard the aircraft are utilized for the determination of engine failures from monitored engine parameters. Computer command speeds and thrust settings for safely flying the aircraft, after a failure has been detected, are displayed to the human pilot or provided to an automatic flight control system.
Prior art jet engine failure detection systems monitor Engine Pressure Ratio (EPR) and Exhaust Gas Temperature (EGT). EPR is derived from pressure sensors mounted in the air inlet and the exhaust nozzle of the engine. The ratio of these two pressures is a direct measure of the thrust of the engine, where EGT is derived from a temperature sensor mounted in the exhaust nozzle of the engine.
Systems utilizing these parameters for jet engine failure detection have numerous short comings. EPR cannot be used during an aircraft's decent, because the EPR during this phase of flight is substantially equal to the EPR of the failed engine. Additionally, the EPR while an aircraft is cruising is small, making the determination of EPR level at which an engine may be considered to have failed, during this phase of flight, extremely difficult. Further, the air quality and temperature in the operating regions may cause pressure sensors positioned in the air inlet to become clogged with ice or debris, thereby producing erroneous measurements.
Measuring EGT also does not provide reliable jet engine failure indications. As for example, the EGT of an engine on fire remains near that of an normally operating engine for an appreciable time period after the start of the fire. Further, since residual heat is dissipated into the atmosphere the EGT, of an engine that has failed for reasons other than fire, is an exponential decay exhibiting a long time constant. Consequently, a significant time lapse may occur before an alarm is given. Another deficiency of the EGT method results from the fact that different models of the same engine may exhibit appreciably different EGTs at idential thrust settings, thereby creating the possibility of a false failure indication.
The limitations inherent in the use of EPR and EGT engine parameters cause failed engine detection to be flight phase dependent; i.e., a different set of criteria must be used for takeoff, cruise, descent etc. Engine failure detection systems utilizing these measurements exhibit high probabilities of erroneous failure detection when the aircraft transitions from one phase of flight to another, as per example, as going from climb to cruise.
Most modern day turbojet or turbofan engines utilize two rotor shafts, the first, a low speed rotor, rotating at N.sub.1 revolutions per minute and the second, a high speed rotor, rotating at N.sub.2 revolutions per minute. A compressor stage, for compressing incoming atmospheric air, is positioned on each rotor shaft near the air inlet end, while a turbine is mounted on the rotor shaft at the end opposite the compressor. As the exhaust gas exits the engine it passes through the turbine blades causing the rotor to rotate, thereby driving the compressor section. Generally rotor angular velocity is expressed as a precentage of the maximum allowable rotational velocity. The determination of the high and low speed rotor angular velocities allows engine failure detection to be completely independent of the flight phase, thus permitting the use of constant engine failure criteria for all flight phases. The rotation velocities N.sub.1 and N.sub.2 are interrelated, one being determinable from the other, for any thrust setting, by a well defined function. This relationship may be utilized to compare the computed and measured shaft rotation speeds for an engine. Additionally, the shaft rotation speed on one engine may be compared to the shaft rotation speed of all other engines mounted on the aircraft.
Other turbojet, turbofan engine characteristics may be calculated or measured for the determination of engine failure. In turbojet, turbofan engines rotors of a failed engine, that are not jammed, will rotate due to the forward motion of the aircraft. This rotation, known as windmilling, has an angular velocity which is a linear function of the aircraft Mach number, thereby providing an additional parameter for detection of a failed engine.